It is generally understood that in certain process applications more than two pipelines may sometimes be connected for flow-mixing or flow-splitting applications. For example, three pipelines may be connected to provide a general converging (flow-mixing) or diverging (flow-splitting) service. In such applications, a three-way valve may be used. Generally, three-way valves have three flow connections. For example, two flow connections may be inlets with one outlet for mixing inlet fluid flow, or there may be one inlet and two outlets for splitting outlet fluid flow.
With reference to FIG. 1, an example of a conventional three-way control valve 10 for a flow-splitting application will be described. The three-way control valve 10 includes a valve body 12, a control member 14, and an actuator 16 (shown in partial cross-section). The valve body 12 defines a flow path 18 for a fluid. As illustrated, this flow path 18 includes a single inlet 18a and two diverging outlets 18b, 18c. In an alternative three-way control valve (not shown), this flow path may include a single outlet and two converging inlets. In either configuration, the control member 14 is disposed within the valve body 12 and adapted for vertical displacement to selectively control the flow of the fluid through flow path 18. The actuator 16 (shown in partial cross-section) is operably coupled to the control member 14 to position the control member 14 in response to some signal or condition. In addition to these general elements, the conventional three-way control valve 10 illustrated in FIG. 1 includes a bonnet 20 disposed between the actuator 16 and the valve body 12. The bonnet 20 serves to couple the actuator 16 to the valve body 12 and provide support, as well as a seal, around the control member 14. Conventionally, the bonnet 20 includes a packing bore 22 for containing a packing assembly 24 to seal around the control member 14.
The control member 14 includes a stem 26 coupled to a valve plug 28. The stem 26 is slidably disposed through the bonnet 20 and operably connected to the actuator 16. The valve plug 28 is disposed within the flow path 18 of the valve body 12 such that it can selectively control the amount of fluid flowing through the flow path 18. The valve plug 28 includes an upper seating surface 28a and a lower seating surface 28b. While in either an upper or lower seated position, the valve plug 28 must be capable of closing the outlet 18b or 18c to which the fluid is to be prevented from flowing. To achieve this, the conventional three-way valve assembly 10 illustrated in FIG. 1 further includes a cage 30 disposed in the valve body 12 having an upper valve seat 32a formed adjacent to an upper window 12a within the cage 30 and a lower valve seat 32b formed within a clamped seat 31 adjacent to the inlet 18a. The cage 30 is fixedly disposed in the flow path 18 of the valve body 12 and provides fluid control through the upper cage windows 12a to the upper outlet 18c and the lower cage windows 12b to the lower outlet 18b as the valve plug 28 moves along an axis defined by the valve stem 26. To restrict flow from the lower outlet 18b depicted in FIG. 1, the lower seating surface 28b of the valve plug 28 sealingly engages the seating surface 32b of the clamped seat 31 when in the lower seated position. Similarly, to restrict flow from the upper outlet 18c depicted in FIG. 1, the upper seating surface 28a of the valve plug 28 sealingly engages the upper seating surface 32a of the valve cage 28 when in the upper seated position.
It should be appreciated by one of ordinary skill in the art that the actuator 16 is adapted to move the valve plug 28 between a lower seated position and an upper seated position in such a manner that flow may occur in various proportions through both the lower and the upper outlets 18b, 18c when the valve plug 28 is in an intermediate position between the upper and lower valve seats 32a, 32b of the cage 30 and the clamped seat 31, respectively.